Method for producing high-porosity vitrified grinding stone

ABSTRACT

A method for producing a high-porosity vitrified grinding stone that has a plurality of pores communicating with each other. The method includes: (a) a grinding-stone-material preparing step of obtaining a grinding-stone raw material slurry that is a mixture fluid of abrasive grains, a vitrified bond, a gellable water-soluble polymer and a water; (b) a molding step of obtaining a molded body, by gelling the grinding-stone raw material slurry with use of a molding mold; (c) a freeze vacuum drying step of generating a plurality of frozen particles inside the molded body by freezing the molded body, and placing the molded body under a vacuum state, so as to sublimate the frozen particles generated inside the molded body for thereby drying the molded body; and (d) a firing step of obtaining the high-porosity vitrified grinding stone, by binding the abrasive grains with the vitrified bond by firing the molded body.

TECHNICAL FIELD

The present invention relates to a method for producing a vitrified grinding stone that has a high porosity due to presence of pores communicating with each other.

BACKGROUND ART

In general, for grinding a semiconductor wafer, there is proposed a high-porosity vitrified grinding stone in which abrasive grains are bound by a vitrified bond to increase an abrasive-grain coercive force and in which a high porosity such as a porosity of 75-95 volume% is established to advantageously provide a self-sharpening effect of the abrasive grains. A vitrified grinding stone disclosed in each of Patent Documents 1 and 2 is an example of such a vitrified grinding stone.

In such a vitrified grinding stone having the high porosity, the high porosity provides the self-sharpening effect of the abrasive grains for thereby increasing a grinding performance, and the high porosity is established by independent pores that provide the grinding stone with a high strength, so that it is possible to advantageously perform a grinding operation with sufficient grinding pressure.

By the way, the high-porosity vitrified grinding stone as disclosed in the Patent Document 1 is produced by pressing a grinding-stone raw material, which is made by kneading and mixing an organic pore-forming agent such as polystyrene particles into abrasive grains and vitrified bond, to form a molded body, and then burning the organic pore-forming agent by firing the molded body. Therefore, in the high-porosity vitrified grinding stone obtained by firing the molded body, most of the pores are the independent pores, i.e, closed pores. Thus, there is a case in which swarf generated in the grinding operation is accumulated in the independent pores thereby making it impossible to satisfactorily obtain the grinding performance.

On the other hand, in Patent Document 3, there is proposed a method of producing a high-porosity vitrified grinding stone having a high porosity of 50-98 volume%, by obtaining a meringue-like foam material by mixing abrasive grains, a vitrified bond, a solidifying agent (gelling agent), a water and a surface-active agent that is used in place of the pore-forming agent, then preparing a molded body by cooling the foam material in a molding mold, then firing the molded body that has been dried, and then immersing the molded boy into a liquid resin so as to cover bond bridges serving as shells surrounding pores, with resin coating layers, for thereby increasing the strength.

Prior Art Documents Patent Documents

-   Patent Document 1: Japanese Unexamined Patent Application     Publication No. 2006-001007 -   Patent Document 2: Japanese Unexamined Patent Application     Publication No. 2017-080847 -   Patent Document 3: Japanese Unexamined Patent Application     Publication No. 2007-290101

SUMMARY OF THE INVENTION Object to Be Achieved by the Invention

However, in the vitrified grinding stone produced by the above-described producing method, the pores are not independent pores, so that there is a case in which the molded body shrinks due to a deformation phenomenon of the bond bridges that is progressed in process of drying of the molded body molded from the meringue-like foam material, so that it is difficult to produce a product having a stable shape.

The present invention was made in view of the background discussed above. It is therefore an object of the present invention to provide a method for producing a vitrified grinding stone having a high porosity due to presence of pores communicating with each other, wherein the method is capable of stably producing the vitrified grinding stone.

Measures for Solving the Problem

Having made various studies under the above-described situation, the present inventors and their colleagues found that it is possible to obtain a vitrified grinding stone which exhibits a sufficient strength in a grinding operation and which has a high porosity due to presence of pores communicating with each other, by obtaining a grinding-stone raw material slurry in which a gelling agent is dissolved and abrasive grains, a vitrified bond and a water are mixed, then preparing a molded body by gelling the grinding-stone raw material slurry in a molding mold, then generating a multiplicity of frozen particles inside the molded body by freezing the molded body, then sublimating the multiplicity of generated frozen particles under a vacuum state so as to form the communicating pores in the molded body, and then firing the porous molded body. The present invention was made based on this finding.

That is, the gist of the present invention is (1) a method of producing a high-porosity vitrified grinding stone that has a plurality of pores communicating with each other, wherein the method includes: (2) a grinding-stone-material preparing step of obtaining a grinding-stone raw material slurry that is a mixture fluid of abrasive grains, a vitrified bond and a water, such that a gellable water-soluble polymer is dissolved in the mixture fluid; (3) a molding step of obtaining a molded body, by gelling the grinding-stone raw material slurry with use of a molding mold; (4) a freeze vacuum drying step of generating a plurality of frozen particles inside the molded body by freezing the molded body after the molding step, and placing the molded body under a vacuum state, so as to sublimate the frozen particles generated inside the molded body for thereby drying the molded body; and (5) a firing step of obtaining the high-porosity vitrified grinding stone, by binding the abrasive grains with the vitrified bond by firing the molded body after the freeze vacuum drying step.

Effects of the Invention

According to the method of producing the high-porosity vitrified grinding stone of the present invention, at the freeze vacuum drying step, with the molded body being placed under the vacuum state, the frozen particles generated inside the molded body are sublimated whereby the molded body is dried so that the plurality of pores communicating with each other are formed after the frozen particles are sublimated. Thus, shrinkage of the molded body is suppressed, and it is possible to stably produce the vitrified grinding stone having the high porosity due to presence of the plurality of pores communicating with each other.

It is preferable that the pores are formed in places in which the frozen particles had been present in the grinding-stone raw material slurry, after the frozen particles have been sublimated at the freeze vacuum drying step. The pores thus formed are not eliminated so that it is possible to suppress the shrinkage of the molded body.

Further, it is preferable that, at the freeze vacuum drying step, the frozen particles are generated in the grinding-stone raw material slurry which is gelled and which constitutes the molded body, whereby the abrasive grains and the vitrified bond are gathered to base portions surrounding the frozen particles, and that, at the firing step, the base portions are fired whereby bond bridges, which are shells that surround the pores, are formed from the base portion. Thus, the strength of the bond bridges is increased even without the bond bridges being covered by reinforcing resin coating layers, so that the vitrified grinding stone can perform a grinding operation although having the high porosity.

Further, it is preferable that a pore volume ratio of the high-porosity vitrified grinding stone is 65-90 volume%. Thus, since the high-porosity vitrified grinding stone has the pore volume ratio of 65-90 volume%, it is possible to obtain both of a grinding efficiency and a grinding stone strength.

Further, it is preferable that a specific gravity of the high-porosity vitrified grinding stone is 0.34-1.48. Thus, the high-porosity vitrified grinding stone having the relatively small specific gravity of 0.34-1.48 can be obtained.

Further, it is preferable that the abrasive grains have a median grain diameter (median size) that is smaller than a thickness of the bond bridges constituting the shells of the pores. Thus, since the abrasive grains are significantly smaller than the thickness of the bond bridges corresponding to the shells of the pores, the bond bridges have a locally non-porous vitrified grinding stone structure so as to increase the strength and the grinding performance of the high-porosity vitrified grinding stone, and a surface roughness suitable for grinding a semiconductor wafer can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1 ] A perspective view showing a cup grinding stone including a base metal and high-porosity vitrified grinding stones fixed to the base metal, wherein each of the high-porosity vitrified grinding stones is produced by a method for producing the high-porosity vitrified grinding stone, which is an embodiment of the present invention.

[FIG. 2 ] A process chart explaining a major part of the method of producing the high-porosity vitrified grinding stone shown in FIG. 1 .

[FIG. 3 ] A set of views schematically showing change of structure in a molded body of the high-porosity vitrified grinding stone in producing process shown in FIG. 2 , wherein (a) shows a state in which a grinding-stone raw material slurry has been prepared at a grinding-stone-material preparing step, (b) shows a state in which frozen particles have been generated inside the molded body by freezing made at a freeze vacuum drying step, (c) shows a state in which the frozen particles have been sublimated inside the molded body by vacuum drying made at the freeze vacuum drying step, and (d) shows a state in which the molded body has been fired at a firing step.

[FIG. 4 ] A view showing circular test pieces (molded bodies), wherein the left-side circular test piece is in a case in which drying has been made in substantially the same manner as in the freeze vacuum drying step of FIG. 2 , and the right-side circular test piece is in a case in which a normal-pressure drying has been made.

[FIG. 5 ] A view showing optical micrographs, wherein the left-side optical micrograph shows in enlargement a pore structure of the molded body before the firing step of FIG. 2 , and the right-side optical micrograph shows in enlargement the pore structure of the molded body after the firing step of FIG. 2 .

[FIG. 6 ] An SEM photograph showing in enlargement the pore structure of the molded body after the firing step of FIG. 2 .

[FIG. 7 ] An SEM photograph showing in further enlargement the pore structure of the molded body after the firing step of FIG. 2 .

[FIG. 8 ] A view showing major data of a grinding stone structure and a grinding ratio of each of samples 11 and 12, so as to compare with samples 1-10 that have been produced substantially the same process of FIG. 2 .

[FIG. 9 ] A view showing samples 1-12 of FIG. 8 in a two-dimensional coordinates having a horizontal axis representing Vg/Vb and a vertical axis representing a specific gravity ρ.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, there will be described an embodiment of the present invention, in detail with reference to the drawings. It is noted that figures of the drawings are simplified or deformed as needed, and each portion is not necessarily precisely depicted in terms of dimension ratio, shape, etc.

Embodiment

FIG. 1 is a perspective view showing a cup grinding stone 14 including a disk-shaped base metal 12 made of metal such as aluminum, and a plurality of segment grinding stones 10 fixed to a lower surface of the base metal 12 such that the segment grinding stones 10 are contiguous to each other and arranged in an annular manner along an outer periphery of the lower surface of the base metal 12, wherein each of the segment grinding stones 10 is an example of a high-porosity vitrified grinding stone as an embodiment of the present invention. The segment grinding stones 10 have respective grinding surfaces 16 that are located on a lower side of an outer peripheral portion of the base metal 12 and contiguous to each other and arranged in an annular manner.

The base metal 12 is constituted by a disk-shaped thick plate made of the metal. With the base metal 12 being attached to a spindle of a grinding machine (not shown), the cup grinding stone 14 is to be driven and rotated. The cup grinding stone 14 has an outside diameter of about 250 mm. Each of the segment grinding stones 10 has a width of about 3 mm and a thickness of about 5 mm. When the base metal 12 is rotated, the segment grinding stones 10 are brought into sliding contact at the respective grinding surfaces 16 with a work material such as silicon wafer, so as to grind or polish the work material to a flat surface shape, for thereby machining a thickness of the work material.

The segment grinding stone 10 is produced, for example, in the producing process shown in FIG. 2 . At a grinding-stone-material preparing step P1, a grinding-stone raw material slurry 18 for the segment grinding stone 10 is prepared. The grinding-stone raw material slurry 18 contains diamond abrasive grains 20 as examples of super abrasive grains, a vitrified bond 22, a water-soluble polysaccharide gelling agent 24 functioning as a primary binder and a water 26 functioning as a pore-forming agent, which are mixed at respective mixing ratios of 17 mass% of the diamond abrasive grains, 15 mass% of the vitrified bond 22, 3 mass% of the water-soluble polysaccharide gelling agent 24, and 65 mass% of the water 26, such that the grinding-stone raw material slurry 18 is prepared to have a liquidity, with the water-soluble polysaccharide gelling agent 24 being dissolved into the water 26 by heating at a temperature of, for example, about 90° C., which is higher than a melting temperature. It is noted that the water-soluble polysaccharide gelling agent 24 corresponds to “gellable water-soluble polymer” in the present embodiment.

FIG. 3 (a) schematically shows the grinding-stone raw material slurry 18. In FIG. 3 (a), the water-soluble polysaccharide gelling agent 24 dissolved into the water 26 constitutes fibrous polysaccharide molecules, and is schematically shown in FIG. 3 (a). These polysaccharide molecules (water-soluble polypeptides gelling agent 24) are entangled with each other and stick to each other to form a network structure, and lots of microscopic spaces for storing the water 26 are formed. Further, the polysaccharide molecule (water-soluble polysaccharide gelling agent 24) being entangled with each other, when the heated temperature of the water-soluble polysaccharide gelling agent 24 is lowered, the liquidity of the water-soluble polysaccharide gelling agent 24 is lost, and the water-soluble polysaccharide gelling agent 24 is gelled. It is noted that, as the water-soluble polysaccharide gelling agent 24, it is possible to use, for example, curdlan, tamarind seed gum, kitan sang gum + locust bean gum, sodium alginate, gelangum, pectin, carrageenan, gelatin and agar.

The vitrified bond 22 has a composition of, for example, 50-54 weight% of SiO₂, 13-15 weight% of Al₂O₃, 17.5-20.5 weight% of B₂O₃, 0.7-6.5 weight% of RO (that is at least one kind of oxide selected from CaO, MgO, BaO and ZnO), 0.0-9.0 weight% of R₂O (that is at least one kind of oxide selected from Li₂O, Na₂O and K₂O) and 0.7-1.3 weight% of P₂O₅.

At a subsequent molding step P2, the grinding-stone raw material slurry 18, which is in a heated state and has the liquidity, is poured into a molding cavity inside the molding mold, wherein the molding cavity has a predetermined shape, e.g., a shape substantially the same as the segment grinding stone 10 and slightly larger than the segment grinding stone 10. Then, with the temperature being reduced to a normal temperature or another temperature that is not higher than a melting temperature of the water-soluble polysaccharide gelling agent 24, the grinding-stone raw material slurry 18 having the liquidity is gelled, namely, solidified, whereby a molded body 28 is obtained, and the molded boy 28 is taken out from the molding mold. FIG. 3 (a) shows a state in which the grinding-stone raw material slurry 18 has been gelled.

Then, at a freeze vacuum drying step P3, the molded body 28 is put into a chamber of a vacuum freeze dryer. Thus, with the molded body 28 being frozen at a predetermined freezing temperature that is not higher than -15° C., for example, a plurality of frozen particles 30 each having a predetermined size is precipitated from the water 26 in the molded body 28. The frozen particles 30 are maintained for a predetermined freezing time whereby each of the frozen particles 30 is caused to grow to a predetermined size. FIG. 3 (b) schematically shows this frozen state. In this frozen state, the diamond abrasive grains 20 and particles of the vitrified bond 22 are driven to interfaces of the frozen particles 30. That is, the diamond abrasive grains 20 and the vitrified bond 22 are gathered to base portions 34 that surround the frozen particles 30.

Then, at the freeze vacuum drying step P3, the molded body 28 is vacuum-dried at 35° C., for example, such that the plurality of frozen particles 30 in the molded body 28 are slowly sublimated, with the molded body 28 being placed under a vacuum state with a predetermined vacuum value (e.g., 10 Pa) that is lower than 610 Pa, for a predetermined time. With the plurality of frozen particles 30 being sublimated, a plurality of pores 32 are formed in places in which the frozen particles 30 has been positioned. FIG. 3 (c) schematically shows this state. In this state, the diamond abrasive grains 20 and the particles of the vitrified bond 22 are dispersed and incorporated in the fibrous polysaccharide molecule (water-soluble polysaccharide gelling agent 24) that surrounds the pores 32. The plurality of frozen particles 30 inside the molded body 28 are brought into contact with each other in process of growth of the frozen particles 30, so that most of the pores 32, which are formed in place of the sublimated frozen particles 30, are pores communicating with each other.

FIG. 4 is a photograph showing circular test pieces as molded bodies that were subjected to the same steps until each of the frozen particles was caused to grow to the predetermined size with the molded bodies being frozen at the predetermined freezing temperature for the predetermined freezing time, wherein a left-side molded body as one of the molded bodies shown in left side in FIG. 4 was subjected to the vacuum drying in the frozen state, while a right-side molded body as the other of the molded bodies shown in right side in FIG. 4 was subjected to the normal-pressure drying at 50° C. The left-side molded boy subjected to the freezing vacuum drying did not lose its shape and had a strength to be easily grasped by hand. On the other hand, the right-side molded body subjected to the normal-pressure drying had a large collapse in its shape, making it impossible to produce the high-porosity vitrified grinding stone.

At a firing step P4, the molded body 28 in which the plurality of pores 32 are formed is fired at a 500-1000° C. as an example of a firing temperature that is not lower than a softening temperature of the vitrified bond 22. With the molded body 28 being fired, the fibrous polysaccharide molecule (water-soluble polysaccharide gelling agent 24) surrounding the pores 32 is burnt and at the same time the diamond abrasive grains 20 and the vitrified bond 22 are sintered to form bond bridges 36 surrounding the pores 32, whereby the segment grinding stone 10 as the high-porosity vitrified grinding stone is obtained. FIG. 3 (d) schematically shows this state.

The bond bridges 36 have a thickness of about 10 µm, and hold the diamond abrasive grains 20 having a median grain diameter of a few µm. Each of the bond bridges 36 is constituted by a dense structure like a non-porous vitrified grinding stone having no pore. Thus, the strength of the bond bridges 36 is increased even without the bond bridges 36 being covered by reinforcing resin coating layers. The median grain diameter of the diamond abrasive grains 20 corresponds to a median size (median diameter) defined by Japanese Industrial Standards (JIS Z 8825:2013), and is a value of D50 by volume conversion.

FIG. 5 is a view showing optical micrographs, wherein the left-side optical micrograph shows in enlargement a pore structure of the molded body 28 before the firing step P4, and the right-side optical micrograph shows in enlargement the pore structure of the molded body 28 after the firing step P4, i.e., the pore structure of the high-porosity vitrified grinding stone 10. As is clear from FIG. 5 , any change of the pore structure of the molded body 28 caused by the firing step P4 is not seen, so that the pore structure before the firing step P4 is maintained after the firing step P4.

FIG. 6 is an SEM photograph showing in further enlargement the pore structure of the molded body 28 after the firing step P4 (i.e., the pore structure of the high-porosity vitrified grinding stone 10). In FIG. 6 , white lines represent the bond bridges 36 surrounding the pores 32. FIG. 7 is an SEM photograph showing in further enlargement the bond bridges 36. As shown in FIG. 7 , the bond bridges 36 has the dense structure. Fine particles on the bond bridges 36 are the diamond abrasive grains 20. The diamond abrasive grains 20 has the median grain diameter that is not larger than 1/50 of the thickness of the bond bridges 36.

Referring back to FIG. 2 , at a bonding/finishing step P5, the plurality of segment grinding stones 10, each of which has been fired at the firing step P4, are bonded to the base metal 12, as shown in FIG. 1 . Then, the segment grinding stones 10 bonded to the base metal 12 is finished by using a dresser.

Hereinafter, the median grain diameter of the diamond abrasive grains, a ratio (= Vg/Vb) of a volume ratio Vg (volume%) of the diamond abrasive grains to a volume ratio Vb (volume%) of the vitrified bond, a specific gravity ρ, a pore volume ratio Vp (volume%) and a grinding ratio GR of samples 1-10 (example products) that are chip-shaped vitrified grinding stones (40 mm×3 mm×5 mm) are shown in FIG. 8 , wherein the samples 1-10 were obtained by the present inventors and their colleagues through a process substantially the same at the process shown in FIG. 2 , with various ratios of the diamond abrasive grains, vitrified bond, water-soluble polysaccharide gelling agent and water. Further, samples 11 and 12 (comparative example products) that are chip-shaped vitrified grinding stones (40 mm×3 mm×5 mm) as well as the samples 1-10 are shown in FIG. 8 , wherein the samples 11 and 12 were preprepared through a process in which the pores were formed with use of conventional pore-forming agents. Further, FIG. 9 is a view showing the samples 1-12 of FIG. 8 that are plotted in a two-dimensional coordinates having a horizontal axis representing the above-described ratio Vg/Vb and a vertical axis representing the specific gravity ρ. The specific gravity ρ of each of the grinding stones is a value obtained by dividing a mass of the grinding stone by a grinding stone volume that is obtained from dimensions of the grinding stone. The volume ratio Vg of the abrasive grains is a value obtained by dividing an abrasive grain volume, which is obtained by dividing a mass of the abrasive grains by a specific gravity of the abrasive grains, by the grinding stone volume. The volume ratio Vb of the bond is a value obtained by dividing a bond volume, which is obtained by dividing a mass of the bond by a specific gravity of the bond, by the grinding stone volume. The pore volume ratio Vp is a value obtained by dividing a volume of the pores by the grinding stone volume.

As shown in FIGS. 8 and 9 , the samples 1-10 are clearly different from the samples11 and 12, particularly, in terms of the pore volume ratio Vp and the grinding ratio GR. The pore volume ratio Vp of the samples 1-10 is 65-90 volume%, so that the pore volume ratio Vp of the samples 1-10 is larger than that of the samples 11 and 12. The grinding ratio GR of the samples 1-10 is 11-750, so that the grinding ratio GR of the samples 1-10 is remarkably larger than that of the samples 11 and 12.

Hereinafter, grinding tests 1 and 2 will be described in details, wherein the grinding stones of the sample 9 (example product) and the sample 11 (comparative example product) were used in the grinding test 1, and the grinding stones of the sample 3 (example product) and the sample 12 (comparative example product) were used in the grinding test 2. The grinding test 2 was performed for an expected rough/finish machining. The grinding test 1 was performed for an expected finish machining.

Grinding Test 1 Composition of Vitrified Bond

Including SiO₂:51.5 mass%, Al₂O₃:14.7 mass%, B₂O₃:18.9 mass%, Na₂O:3.9 mass%, K₂O:3.8 mass%, MgO:2.0 mass%, CaO:1.9 mass%, BaO:0.7 mass% and P₂O₅:0.9 mass%.

Mixing for sample 9 Diamond abrasive grains of median grain diameter of 0.2 µm :17 mass% Vitrified bond :15 mass% Water-soluble polysaccharide gelling agent (agar) : 3 mass% Water : 65 mass%

Method of Producing Sample 9

The chip-shaped vitrified grinding stone (sample 9), which has substantially the same shape as the segment grinding stone 10, was prepared from the above-described mixing through the producing process of FIG. 2 , such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of the diamond abrasive grains is 7.0 volume%, the volume ratio Vb of the vitrified bond is 8.6 volume%, the ratio Vg/Vb is 0.8, the pore volume ratio Vp is 84.4 volume%, and the specific gravity is 0.46 g/cm³.

Method of Producing Sample 11

The chip-shaped vitrified grinding stone (sample 11), which has substantially the same shape as the segment grinding stone 10, was prepared through a conventional process forming pores using a pore-forming agent, such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of diamond abrasive grains having a median grain diameter of 0.2 µm is 27.2 volume%, the volume ratio Vb of the vitrified bond substantially the same as that in the sample 9 is 17.3 volume%, the ratio Vg/Vb is 1.6, the pore volume ratio Vp is 55.5 volume%, and the specific gravity is 1.55 g/cm³.

Grinding Test Method

The grinding tests were performed to a silicon wafer having a diameter of 12 inches under a grinding condition specified below by using grinding wheels each of which was attached to a vertical plane grinding machine, wherein the vitrified grinding stones of a corresponding one of the samples 9 and 11 were bonded, as shown in FIG. 1 , to a lower surface of a base metal made of aluminum and having an outside diameter of 300 mm in each of the grinding wheels.

Machining condition in grinding test Rotational speed of grinding stone :3000 rpm Rotational speed of table (wafer) :395 rpm Axial feed rate of grinding stone :0.5 µm/sec Machining allowance of wafer :thickness 20 µm

Result of grinding test of sample 9 Amount of wear of grinding stone of sample 9 :1.7 µm Current value during machining :19.0A Surface roughness Ra (JIS B 0601:2013) :1.2 nm

Result of grinding test of sample 11 Amount of wear of grinding stone of sample 11 :35.3 µm Current value during machining :18.9A Surface roughness Ra (JIS B 0601:2013) :1.2 nm

Evaluation of Results of Grinding Tests

The grinding using the sample 9 is not so different from the grinding using the sample 11 in terms of the current value during machining and the surface roughness Ra. However, the amount of wear of grinding stone was remarkably reduced when the sample 9 was used, since the grinding ratio was 11.8 (= 20 µm/1.7 µm) with use of the sample 9 while the grinding ratio was 0.57 (= 20 µm/35.3 µm) with use of the sample 11.

Grinding Test 2 Composition of Vitrified Bond

Including SiO₂:51.5 mass%, Al₂O₃:14.7 mass%, B₂O₃:18.9 mass%, Na₂O:3.9 mass%, K₂O:3.8 mass%, MgO:2.0 mass%, CaO:1.9 mass%, BaO:0.7 mass% and P₂O₅:0.9 mass%.

Mixing for sample 3 Diamond abrasive grains of median grain diameter of 6 µm :21 mass% Vitrified bond :12 mass% Water-soluble polysaccharide gelling agent (agar) : 3 mass% Water : 64 mass%

Method of Producing Sample 3

The chip-shaped vitrified grinding stone (sample 3), which has substantially the same shape as the segment grinding stone 10, was prepared from the above-described mixing through the producing process of FIG. 2 , such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of the diamond abrasive grains is 17.3 volume%, the volume ratio Vb of the vitrified bond is 14.2 volume%, the ratio Vg/Vb is 1.2, the pore volume ratio Vp is 68.5 volume%, and the specific gravity is 0.96 g/cm³.

Method of Producing Sample 12

The chip-shaped vitrified grinding stone (sample 12), which has substantially the same shape as the segment grinding stone 10, was prepared through a conventional process forming pores using a pore-forming agent, such that the prepared vitrified grinding stone has a structure in which the volume ratio Vg of diamond abrasive grains having a median grain diameter of 6 µm is 40.9 volume%, the volume ratio Vb of the vitrified bond substantially the same as that in the sample 3 is 12.3 volume%, the ratio Vg/Vb is 3.3, the pore volume ratio Vp is 47.8 volume%, and the specific gravity is 1.95 g/cm³.

Grinding Test Method

The grinding tests were performed to a silicon wafer having a diameter of 12 inches under a grinding condition specified below by using grinding wheels each of which was attached to a vertical plane grinding machine, wherein the vitrified grinding stones of a corresponding one of the samples 3 and 12 were bonded, as shown in FIG. 1 , to a lower surface of a base metal made of aluminum and having an outside diameter of 300 mm in each of the grinding wheels.

Machining condition in grinding test Rotational speed of grinding stone :2000 rpm Rotational speed of table (wafer) :300 rpm Axial feed rate of grinding stone :40.00 µm/sec Machining allowance of wafer :thickness 150 µm

Result of grinding test of sample 3 Amount of wear of grinding stone of sample 3 :0.2 µm Current value during machining :23.8A Surface roughness Ra (JIS B 0601:2013) :44.4 nm

Result of grinding test of sample 12 Amount of wear of grinding stone of sample 12 :164 µm Current value during machining :14.2A Surface roughness Ra (JIS B 0601:2013) :39.1 nm

Evaluation of Results of Grinding Tests

The grinding using the sample 3 is not so different from the grinding using the sample 12 in terms of the surface roughness Ra, although the current value during machining using the sample 3 is increased as compared with machining using the sample 12. However, the amount of wear of grinding stone was remarkably reduced when the sample 3 was used, since the grinding ratio was 750 (= 150 µm/0.2 µm) with use of the sample 3 while the grinding ratio was 0.91 (= 150 µm/164 µm) with use of the sample 12.

As described above, the method of producing the high-porosity vitrified grinding stone 10 (segment grinding stone 10) of the present embodiment is a method of producing the high-porosity vitrified grinding stone 10 that includes the plurality of pores 32 communicating with each other. The method includes: the grinding-stone-material preparing step P1 of obtaining the grinding-stone raw material slurry 18 that is a mixture fluid of the diamond abrasive grains (abrasive grains) 20, the vitrified bond 22 and the water 26, such that the water-soluble polysaccharide gelling agent 24 is dissolved in the mixture fluid; the molding step P2 of obtaining the molded body 28, by gelling the grinding-stone raw material slurry 18 with use of the molding mold; the freeze vacuum drying step P3 of generating the plurality of frozen particles 30 inside the molded body 28 by freezing the molded body 28 after the molding step P2, and placing the molded body 28 under the vacuum state, so as to sublimate the frozen particles 30 generated inside the molded body 28 for thereby drying the molded body 28; and the firing step P4 of obtaining the high-porosity vitrified grinding stone 10, by binding the abrasive grains 20 with the vitrified bond 22 by firing the molded body 28 after the freeze vacuum drying step P3. Thus, at the freeze vacuum drying step P3, with the molded body 28 being placed under the vacuum state, the frozen particles 30 generated inside the molded body 28 are sublimated whereby the molded body 28 is dried so that the plurality of pores 32 communicating with each other are formed after the frozen particles 30 are sublimated. Thus, shrinkage of the molded body 28 due to elimination of bubbles is not caused, so that it is possible to stably produce the vitrified grinding stone 10 having the high porosity due to presence of the plurality of pores 32 communicating with each other.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, the pores 32 communicating with each other are formed in places in which the frozen particles 30 had been present inside the molded body 28, after the frozen particles 30 have been sublimated at the freeze vacuum drying step P3. The pores 32 thus formed are not eliminated so that it is possible to suppress the shrinkage of the molded body 28.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, at the freeze vacuum drying step P3, the frozen particles 30 are generated inside the molded body 28 in which the grinding-stone raw material slurry 18 that is solidified into a gel, whereby the diamond abrasive grains 20 and the vitrified bond 22 are gathered to the base portions 34 surrounding the frozen particles 30, and, at the firing step P4, the base portions 34 are fired whereby the bond bridges 36 are formed as the shells that surround the pores 32. Each of the bond bridges 36 is constituted by a structure like a non-porous vitrified grinding stone having no pore. Thus, the strength of the bond bridges 36 is increased even without the bond bridges 36 being covered by reinforcing resin coating layers, so that the vitrified grinding stone 10 can perform a grinding operation although having the high porosity.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, the pore volume ratio Vp of the high-porosity vitrified grinding stone 10 is 65-90 volume%. Thus, since the high-porosity vitrified grinding stone 10 has the pore volume ratio Vp of 65-90 volume%, it is possible to obtain both of a grinding efficiency and a grinding stone strength.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, the specific gravity of the high-porosity vitrified grinding stone 10 is 0.34-1.48. Thus, the high-porosity vitrified grinding stone 10 having the relatively small specific gravity of 0.34-1.48 can be obtained.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, the grinding ratio of the high-porosity vitrified grinding stone 10 is 11-750. Thus, since the grinding ratio is a value as high as 11-750, the high-porosity vitrified grinding stone 10 having a durability can be obtained.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, the diamond abrasive grains 20 have the median grain diameter that is smaller than the thickness of the bond bridges 36 constituting the shells of the pores 32. Thus, since the diamond abrasive grains 20 are significantly smaller than the thickness of the bond bridges 36 corresponding to the shells of the pores 32, the bond bridges 36 have a locally non-porous vitrified grinding stone structure so as to increase the strength and the grinding performance of the high-porosity vitrified grinding stone 10, and a surface roughness suitable for grinding a semiconductor wafer can be obtained.

Further, in the method for producing the high-porosity vitrified grinding stone 10 according to the present embodiment, the vitrified bond 22 includes 50-54 weight% of SiO₂, 13-15 weight% of AI₂O₃, 17.5-20.5 weight% of B₂O₃, 0.7-6.5 weight% of RO (that is at least one kind of oxide selected from CaO, MgO, BaO and ZnO), 0.0-9.0 weight% of R₂O (that is at least one kind of oxide selected from Li₂O, Na₂O and K₂O) and 0.7-1.3 weight% of P₂O₅. Thus, it is possible to obtain the vitrified bond 22 having a high strength and suitable for grinding a semiconductor wafer, and to increase the durability of the high-porosity vitrified grinding stone 10.

While the embodiment of the present invention has been described in detail with reference to the drawings, the present invention is not limited to details of the embodiment but may be embodied also in other forms.

For example, in the above-described embodiment, the high-porosity vitrified grinding stone constitutes each of the arc-shaped segment grinding stones 10 that are fixed to the base metal 12. However, the high-porosity vitrified grinding stone may be formed to have a disk or other shape.

Further, in the segment grinding stone 10, a part to be involved in a grinding operation, for example, a grinding stone layer provided in a part including the grinding surface 16, may be constituted by the high-porosity vitrified grinding stone.

Further, in the above-described embodiment, the diamond abrasive grains are employed as the abrasive grain. However, the abrasive grains do not necessarily have to be the diamond abrasive grains, and may be also abrasive grains other than super abrasive grains. Further, although the water-soluble polysaccharide gelling agent is used as the gellable water-soluble polymer in the above-described embodiment, the gellable water-soluble polymer does not necessarily have to be polysaccharide.

It is noted that what has been described above is merely an embodiment of the present invention, and that the present invention may be embodied with various modifications and improvements based on knowledges of those skilled in the art in a range without departing from the spirit of the invention, although the modifications and improvements have not been described by way of examples.

DESCRIPTION OF REFERENCE SIGNS

10: segment grinding stone (high-porosity vitrified grinding stone) 12: base metal 14: cup grinding stone 16: grinding surface 18: grinding-stone raw material slurry 20: diamond abrasive grains (grains) 22: vitrified bond 24: water-soluble polysaccharide gelling agent (gellable water-soluble polymer) 26: water 28: molded body 30: frozen particles 32: pores 34: base portions 36: bond bridges 

1. A method for producing a high-porosity vitrified grinding stone that has a plurality of pores communicating with each other, the method comprising: a grinding-stone-material preparing step of obtaining a grinding-stone raw material slurry that is a mixture fluid of abrasive grains, a vitrified bond and a water, such that a gellable water-soluble polymer is dissolved in the mixture fluid; a molding step of obtaining a molded body, by gelling the grinding-stone raw material slurry with use of a molding mold; a freeze vacuum drying step of generating a plurality of frozen particles inside the molded body by freezing the molded body after the molding step, and placing the molded body under a vacuum state, so as to sublimate the frozen particles generated inside the molded body for thereby drying the molded body; and a firing step of obtaining the high-porosity vitrified grinding stone, by binding the abrasive grains with the vitrified bond by firing the molded body after the freeze vacuum drying step.
 2. The method of the according to claim 1, wherein the pores are formed in places in which the frozen particles had been present in the grinding-stone raw material slurry, after the frozen particles have been sublimated at the freeze vacuum drying step.
 3. The method according to claim 1, wherein, at the freeze vacuum drying step, the frozen particles are generated in the grinding-stone raw material slurry which is gelled and which constitutes the molded body, whereby the abrasive grains and the vitrified bond are gathered to base portions surrounding the frozen particles, and wherein, at the firing step, the base portions are fired whereby bond bridges, which are shells that surround the pores, are formed from the base portion.
 4. The method according to claim 1, wherein a pore volume ratio of the high-porosity vitrified grinding stone is 65-90 volume%.
 5. The method according to claim 1, wherein a specific gravity of the high-porosity vitrified grinding stone is 0.34-1.48.
 6. The method according to claim 3, wherein the abrasive grains have a median grain diameter that is smaller than a thickness of the bond bridges constituting the shells of the pores. 